Electrocyclic Reactions
O
O
O
Me
Me
OPMB
XX
OTBS
Lecture Notes
Key Reviews:
Electrocyclic Reactions:
Comprehensive Organic Synthesis, Vol. 5, p. 699
Nazarov Cyclization (4π-electrocyclization):
S. E. Denmark, Org. React. 1994, 45, 1-158.
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Electrocyclic Reactions, Exercises of Analytical Chemistry
The Woodward-Hoffmann rules predict that photochemical reactions will be precisely complementary to thermal reactions. Thus, what is allowed.
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Electrocyclic Reactions
Lecture Notes
Key Reviews:
Electrocyclic Reactions:
Comprehensive Organic Synthesis, Vol. 5 , p. 699
Nazarov Cyclization ( 4 π - electrocyclization):
S. E. Denmark, Org. React. 1994 , 45 , 1 - 158.
A Series of Observations:
The Birth of the Woodward-Hoffmann Rules and Electrocyclizations
E. Havinga and co-workers, Tetrahedron 1960 , 11 , 276.
E. Havinga and co-workers, Tetrahedron 1961 , 12 , 146.
HO
Me
Me
Me Me
Me
precalciferol
(previtamin D)
HO
Me
Me Me
Me
ergosterol
HO
Me
Me Me
Me
lumisterol
Me
H
Me
H
h ν
A Series of Observations:
The Birth of the Woodward-Hoffmann Rules and Electrocyclizations
E. Havinga and co-workers, Tetrahedron 1960 , 11 , 276.
E. Havinga and co-workers, Tetrahedron 1961 , 12 , 146.
HO
Me
Me
Me Me
Me
precalciferol
(previtamin D)
HO
Me
Me Me
Me
ergosterol
HO
Me
Me Me
Me
lumisterol
Me
H
Me
H
HO
Me
Me Me
Me
Me
H
HO
Me
Me Me
Me
Me
H
pyrocalciferol
isopyrocalciferol
200 °C
h ν
A Series of Observations:
The Birth of the Woodward-Hoffmann Rules and Electrocyclizations
E. Havinga and co-workers, Tetrahedron 1960 , 11 , 276.
E. Havinga and co-workers, Tetrahedron 1961 , 12 , 146.
HO
Me
Me
Me Me
Me
precalciferol
(previtamin D)
HO
Me
Me Me
Me
ergosterol
HO
Me
Me Me
Me
lumisterol
Me
H
Me
H
HO
Me
Me Me
Me
Me
H
HO
Me
Me Me
Me
Me
H
pyrocalciferol
isopyrocalciferol
200 °C
h ν 6 π - conrotatory
electrocyclization
6 π - disrotatory
electrocyclization
Background and Basic Principles
A
B
B
A
A
A
B
B
A
B
B
A
A
B
A
B
Conrotatory versus disrotatory cyclization: how can I differentiate them?
A
B
A
B
A
B
A
B
A
B
B
A
or
B
A
B
A
disrotatory
ring opening
conrotatory
ring opening
conrotatory
ring closing
disrotatory
ring closing
disrotatory
ring opening
disrotatory
ring closing
conrotatory
ring opening
conrotatory
ring closing
A Series of Observations:
The Birth of the Woodward-Hoffmann Rules and Electrocyclizations
E. Havinga and co-workers, Tetrahedron 1960 , 11 , 276.
E. Havinga and co-workers, Tetrahedron 1961 , 12 , 146.
HO
Me
Me
Me Me
Me
precalciferol
(previtamin D)
HO
Me
Me Me
Me
ergosterol
HO
Me
Me Me
Me
lumisterol
Me
H
Me
H
HO
Me
Me Me
Me
Me
H
HO
Me
Me Me
Me
Me
H
pyrocalciferol
isopyrocalciferol
200 °C
h ν 6 π - conrotatory
electrocyclization
6 π - disrotatory
electrocyclization
Background and Basic Principles
A
B
B
A
A
A
B
B
A
B
B
A
A
B
A
B
Conrotatory versus disrotatory cyclization: how can I differentiate them?
A
B
A
B
A
B
A
B
A
B
B
A
or
B
A
B
A
disrotatory
ring opening
conrotatory
ring opening
conrotatory
ring closing
disrotatory
ring closing
disrotatory
ring opening
disrotatory
ring closing
conrotatory
ring opening
conrotatory
ring closing
Background and Basic Principles
Guiding Principle:
Electron donors go outward;
electron withdrawers go inward.
4 π - conrotatory
electrocyclic
ring opening
F
F
CF
3
F
CF
3
F
Δ
F
F
F
CF
3
F
CF
3
F
F
CF
3
F
CF
3
F
F
F
CF
3
CF
3
F
F
h ν
4 π - disrotatory
electrocyclic
ring opening
What happens when the rotating groups are different? Will one product predominate?
Here, the products are
degenerate; i.e., the same
Background and Basic Principles
e thermal photochemical
disrotatory
conrotatory
disrotatory
conrotatory
conrotatory
disrotatory
conrotatory
disrotatory
O
O
O
H
H
O
O
O
H
H
H
H
H
H
h ν
4 π - disrotatory
electrocyclization
O
O
O
H
H
H
H
H
O
O
O
H
H
H
impossible
structure! 4 π - conrotatory
electrocyclization
trans-fused
4 - membered ring
Δ
X
Background and Basic Principles
e thermal photochemical
disrotatory
conrotatory
disrotatory
conrotatory
conrotatory
disrotatory
conrotatory
disrotatory
H
H
h ν
4 π - disrotatory
electrocyclization
Background and Basic Principles
e thermal photochemical
disrotatory
conrotatory
disrotatory
conrotatory
conrotatory
disrotatory
conrotatory
disrotatory
impossible
structure!
c
b
a
d
b
d
a
c
H
H
a
c
b
d
c
b
a
d
b
c
a
d
H
H
a
d
b
c
h ν
6 π - disrotatory
electrocyclization
6 π - conrotatory
electrocyclization
Δ
X
h ν
8 π - conrotatory
electrocyclization
8 π - disrotatory
electrocyclization
Δ
Background and Basic Principles
e thermal photochemical
disrotatory
conrotatory
disrotatory
conrotatory
conrotatory
disrotatory
conrotatory
disrotatory
impossible
structure!
c
b
a
d
b
d
a
c
H
H
a
c
b
d
c
b
a
d
b
c
a
d
H
H
a
d
b
c
h ν
6 π - disrotatory
electrocyclization
6 π - conrotatory
electrocyclization
Δ
X
h ν
8 π - conrotatory
electrocyclization
8 π - disrotatory
electrocyclization
Δ
Background and Basic Principles
e thermal photochemical
disrotatory
conrotatory
disrotatory
conrotatory
conrotatory
disrotatory
conrotatory
disrotatory
O
OH
OH
4 π - conrotatory
electrocyclization
O
Note: Electrocyclic reactions can involve both cationic and anionic intermediates
H
3
PO
4
This reaction
is known as the
Nazarov
cyclization and
was first
discovered
in 1948
Background and Basic Principles
e thermal photochemical
disrotatory
conrotatory
disrotatory
conrotatory
conrotatory
disrotatory
conrotatory
disrotatory
O
OH
H
3
PO
4
OH
O
The Nazarov reaction is highly regioselective based on carbocation stability
S. E. Denmark and co-workers, J. Am. Chem. Soc. 1982 , 104 , 2642.